Fix `rails plugin --help`

Right now if you run the `rails plugin --help` command it fails because rails expects a command in `railties/lib/rails/commands/plugin.rb` that does not exist because the file is named `plugin_new`. This is the error:

```
ruby-2.0.0-p0  ~/documents/projects/tmp/vanilla (master)
$ rails plugin --help
/Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require': cannot load such file -- rails/commands/plugin (LoadError)
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0/lib/rails/commands.rb:49:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
```
Merge pull request #11176 from schneems/schneems/plugin_new_help_fix

Fix `rails plugin --help`